Field of the Invention
The availability of laser sources generating optical pulses of ultrashort duration (i.e. less than 10 picoseconds) has given the possibility to analyze the time dynamics of phenomena in specific media on time scales of picoseconds to attoseconds. These laser pulses are characterized by a broad spectrum of optical frequencies, the frequency range Δv being related to the minimum duration of the pulse Δt (also known as the Fourier Transform Limit) by the approximate relationship Δv≈1/Δt., implying Δv values exceeding 0.1 TeraHertz (THz).
Description of the Related Art
A particularly effective method in the state of the art to achieve the study of time dynamics is the pump and probe technique, wherein an original laser pulse is used to produce two secondary pulses, named respectively pump pulse and probe pulse. The pump pulse excites the medium under study at time t0, and the probe pulse suitably delayed by a time δt with respect to the pump pulse is used to monitor some properties of the medium under study at time t0+δt. Repeating this experiments for different delay times δt yields a kind of slow motion picture of the ultrafast dynamics. For instance, the probe pulse may traverse the medium and measuring its intensity can be used to monitor optical absorption as a function of time. An interesting variation of the pump and probe technique is Terahertz spectroscopy, wherein the pump pulse is sent to an electro-optic generator, to excite an electrical pulse in the THz range of frequencies. Examples of electro-optic generators are a photoconductor with an applied voltage or anon-linear optical material capable of optical rectification. An electro-optic detector is submitted to the probe pulse and to the THz wave, after it has interacted with the medium understudy. This achieves sampling of the THz wave. Such electro-optic detectors produce a signal proportional to the product of the THz wave electric field and the probe pulse energy. An implementation of an electro-optic detector can for instance use the polarization rotation in an electro-optic medium generated by the THz wave field, such rotation being measured by the transmission of the probe pulse through a suitable combination of the electro-optic medium and polarizers. A photoconductor polarized by the THz field and excited by the probe pulse can also be used as a detector.
In general, these measurements involve problems of sensitivity due to the small magnitude of the effects involved and to the short duration of the sampling probe pulse. It is usually necessary to sum the measurement observed over a number of successive laser pulses to obtain a meaningful result. Because measurements will always be subject to drifts of different origins, it is also desirable to vary rapidly the relevant parameter, in the present case the time delay, so that drifts can be minimal during the time of measurement. A desirable measurement condition, is thus to obtain a full measurement of the relevant delay interval in as short a time as possible, then repeat this measurement a sufficient number of times, summing separately the values for each individual δt, in order to improve the signal to noise ratio. From these considerations, it is clear that a scanning method for the delay δt is required with the maximum possible repetition rate. Also, a high precision for the delay is desirable leading to criteria for the stability and the magnitude of the delay difference between successive optical pulses.
In the state of the art, two main scanning methods have been used. The first one uses a mechanical delay line, wherein the delay is controlled by the different in length of two separate optical paths, for the pump and probe pulse respectively. This method leads to notoriously low scanning rates, typically limited to a few tens of Hertz, due to the inertia of the mechanical components. It is thus inadequate for rapid scanning. Faster techniques have been developed based on rotatable mirrors or quickly moving loudspeaker diaphragms, which can attain scan rates of 100 Hz up to a few kHz. However, with a time jitter between scans far exceeding one femtosecond, these techniques are less precise and therefore unsuitable for analyzing processes occurring on few femtosecond and attosecond timescales. In a similar fashion, Piyaket et al. (Programmable ultrashort optical pulse delay using an acousto-optic deflector”, Applied optics, Optical Society of America, Washington, D.C.; US vol. 34, no 8, 10 Mar. 1995, pages 1445-1453) have replaced the rotable mirror by an acousto-optic deflector, eliminating one source of timing jitter, but with remaining issues of mechanical stability and producing a delay scanning in discontinuous steps.
The second method, described for instance in “Bartels et al. 2007” (Bartels, A., R. Cerna, C. Kistner, A. Thoma, F. Hudert, C. Janke, and T. Dekorsy. 2007. “Ultrafast time-domain spectroscopy based on high-speed asynchronous optical sampling.” Review of Scientific Instruments 78 (3): 035107), uses no mechanical delay, but instead relies on two separate laser sources for the pump and probe pulse respectively. These sources each generate a train of pulses, but the pulse repetition frequency is chosen to be slightly different for the pump and probe sources, by an amount df=f2−f1., where f2 and f1 are the probe and pulse repetition frequencies respectively. Hence for each successive pulse/probe pairs, the time difference between pulse and probe will be increased by a value dt=1/f1−1/f2. Using typical values of 100 MHz for f1 and f2 and 10−4 of relative difference df/f, it is possible to obtain 10000 different sampling points, by increments of 1 ps, in the scanning range of amplitude 0 to 10 ns, and a repetition rate of 10 KHz. The same reasoning applied to a 1 GHz repetition rate source yields a range of 1 ns and a repetition rate of 100 KHz. This method is generally referred as asynchronous optical sampling (ASOPS). Besides the complexity and cost associated with the requirement for two sources, the latter method has two disadvantages. First, many studies are interested in delay ranges significantly shorter than 1-10 ns. Consequently, a large part of the data measured is insignificant and lost. Second the synchronization between the two sources is done by electronic means and limited by inherent electronic jitter. The scan precision in the state of the art does not reach values below 50 fs. This is of the same order as the minimum difference of delay between successive pulses.